Of the many mixing elements that have been made, three fluted mixers are of particular interest. These are the mixers of G. LeRoy U.S. Pat. No. 3,486,192, R. B. Gregory and L. F. Street U.S. Pat. No. 3,411,179, and R. G. Dray, U.S. Pat. No. 3,788,612. These mixers are often claimed to have dispersive mixing properties because the barrier clearance within the inner wall of the outer cylinder of the extruder is so small that the material is subjected to a high shear rate, the corresponding shear stress then being large enough to break down particles in the polymer melt. An important assumption in this prior art analysis is that the mixer is filled with plasticized material under compression. For example, in R. G. Dray U.S. Pat. No. 3,788,612, since the material is described as plasticized and forced through increasing resistance through the mixing element, the intention of the patent is to force material, under compression, through this mixing element. The foregoing will be seen to be contrary to the substance of this invention.